The present invention relates to a two-stroke engine, especially as a drive motor in a portable, manually-guided implement such as a power chain saw, a brush cutter, a trimmer, a cut-off machine, or the like.
A two-stroke engine of this type is known from WO 00/11334. Formed in a cylinder is a combustion chamber that is delimited relative to a crankcase by a reciprocating piston. By means of a connecting rod, the piston drives a crankshaft that is rotatably mounted in the crankcase and that drives the tool.
The exhaust gases that result during the combustion in the combustion chamber are withdrawn via an exhaust gas outlet, which is disposed across from an inlet that supplies fresh mixture to the combustion chamber. The inlet forms one end of a storage channel, the other end of which opens out into the crankcase via a window that is controlled by the piston. In this connection, the storage channel is connected with a mixture-forming device that supplies fuel, whereby combustion air is essentially supplied to the crankcase via a crankcase inlet, with the combustion air being transferred to the combustion chamber via a transfer channel.
Such a two-stroke engine utilizes, in a special way, the high pressure level of the exhaust gases for the introduction of a rich fuel mixture into the combustion chamber. Critical in this connection is that the storage channel have such a length that an adequate volume is available for drawing in the necessary rich mixture, and furthermore that an effective introduction of this mixture into the combustion chamber is ensured. For this purpose, the inlet is opened approximately simultaneously with the outlet, so that the exhaust gas that is under high pressure enters the storage channel via the inlet and moves through the storage channel as a pressure wave. Before the pressure wave can reach the other end of the storage channel, the latter is closed by the upwardly moving piston, so that the pressure wave is reflected at the piston skirt and returns. Via the now completely open inlet, the returning pressure wave conveys the mixture stored in the storage channel in a pulse-like manner into the combustion chamber, to which the combustion air that is necessary for the combustion is supplied via the combustion channel.
To store a fuel in the storage channel, a fuel supply is connected in the region of the inlet. This fuel supply requires a check valve so that during the introduction of the rich mixture by the reflected exhaust gas pulse, a return flow of fuel via the mixture channel is prevented. Such a check valve influences the entire system due to its characteristics, so that a satisfactory supply of fuel for fuel storage in the storage channel cannot always be achieved in all operating ranges of the internal combustion engine.
It is therefore an object of the present invention to improve an internal combustion engine of the aforementioned general type in such a way that a sufficient supply of fuel into the storage channel, in a manner free of disruption, is ensured under all operating conditions.